Open Access

Go contributes to olfactory reception in Drosophila melanogaster

BMC Physiology20099:22

https://doi.org/10.1186/1472-6793-9-22

Received: 12 May 2009

Accepted: 28 November 2009

Published: 28 November 2009

Abstract

Background

Seven-transmembrane receptors typically mediate olfactory signal transduction by coupling to G-proteins. Although insect odorant receptors have seven transmembrane domains like G-protein coupled receptors, they have an inverted membrane topology and function as ligand-gated cation channels. Consequently, the involvement of cyclic nucleotides and G proteins in insect odor reception is controversial. Since the heterotrimeric Goα subunit is expressed in Drosophila olfactory receptor neurons, we reasoned that Go acts together with insect odorant receptor cation channels to mediate odor-induced physiological responses.

Results

To test whether Go dependent signaling is involved in mediating olfactory responses in Drosophila, we analyzed electroantennogram and single-sensillum recording from flies that conditionally express pertussis toxin, a specific inhibitor of Go in Drosophila. Pertussis toxin expression in olfactory receptor neurons reversibly reduced the amplitude and hastened the termination of electroantennogram responses induced by ethyl acetate. The frequency of odor-induced spike firing from individual sensory neurons was also reduced by pertussis toxin. These results demonstrate that Go signaling is involved in increasing sensitivity of olfactory physiology in Drosophila. The effect of pertussis toxin was independent of odorant identity and intensity, indicating a generalized involvement of Go in olfactory reception.

Conclusion

These results demonstrate that Go is required for maximal physiological responses to multiple odorants in Drosophila, and suggest that OR channel function and G-protein signaling are required for optimal physiological responses to odors.

Background

Most animals rely on olfaction for foraging, predator and toxin avoidance, and social interactions. Odorants are detected by 7-transmembrane receptors, which normally transduce olfactory signaling by activating G-proteins. However, recent work in the fruit fly Drosophila melanogaster demonstrates that insect odorant receptors (ORs) act as ligand gated [1, 2] and cyclic nucleotide gated [2] cation channels, and thus do not function as traditional G-protein coupled receptors. The Gα protein(s) responsible for inducing the production of cyclic nucleotides that activate cation channels formed by OR-complexes have not been identified, although Gq has been implicated in Drosophila olfactory transduction [3]. Another Gα protein, Go, is expressed in the odorant receptor neurons (ORNs) of antenna from Drosophila, the silk moth Bombyx mori, and the mosquito Anopheles gambae, suggesting the functional involvement of Go in insect olfaction [47]. Although definitive immunohistochemical proof for dendritic localization of Go in olfactory sensilla is lacking, previous studies could not rule out the possibility of Go expression in ORN dendrites.

In Drosophila, the S1 subunit of pertussis toxin (PTX) selectively ADP-ribosylates Go, thereby inhibiting Go signaling [8, 9]. We have employed existing and newly developed tools for controlling the spatial and temporal expression of PTX to investigate how Go inactivation affects physiological responses to odorants [10, 11]. Loss of Go signaling in ORNs reduced the amplitude and enhanced the termination of EAG responses, and decreased odor-induced spike frequency in individual ORNs independent of odor type or concentration. These results demonstrate that Go is involved in modulating olfactory responses in Drosophila.

Results and Discussion

To determine whether Go signaling mediates olfactory responses, EAG measurements were carried out on flies in which the widespread olfactory receptor neuron driver Or83b-Gal4 was used to drive UAS-PTX in ORNs [12]. Conditional expression of PTX was achieved using the Gal80ts20 TARGET system; at 18°C, functional GAL80ts20 binds to and inhibits GAL4 and at 32°C GAL80ts20 is inactivated thus allowing PTX expression [13]. At 32°C, Or83b promoter driven GAL4 was free to drive the transcription of PTX and inactivate Drosophila Go [10]. As a result, Gal80ts20/Or83b-Gal4; UAS-PTX/+ flies, which show a ~12 mV EAG amplitude to 10-4 ethyl acetate at 18°C, produce a significantly (p < 0.0001) decreased EAG amplitude of ~7 mV at 32°C (Fig 1, Fig 2A). This result demonstrates that PTX-sensitive Go is needed for high amplitude EAG responses, suggesting that Go is involved in generating receptor potential. To insure that the observed decrease in EAG amplitude did not arise from cell damage and/or cell death, we placed temperature-treated flies to 18°C for 24 hours and measured EAG responses. These flies regained normal EAG amplitude of ~12 mV, demonstrating that the effect of PTX is reversible (Fig 2A). Moreover, EAG responses evoked by 10-4 ethyl acetate in Gal80ts20/+; UAS-PTX/+ and Or83b-Gal4/+ control strains did not show a decreased (p > 0.5) amplitude when the temperature was increased from 18°C to 32°C (Fig 2A), thus decreased amplitude does not result from an increased temperature. Temperature did induce a moderate increase in EAG amplitude in Gal80ts20/+; UAS-PTX/+ control flies, but this is likely due to the Gal80ts20 transgene genetic background since Gal80ts20/+ flies displayed a modest increase in EAG amplitude when temperature was increased to 32°C (Additional file 1).
Figure 1

PTX reduces the amplitude of ethyl acetate induced EAG responses. EAG traces evoked by the application of 10-4 ethyl acetate in Gal80ts20/Or83b-Gal4; UAS-PTX/+ flies at temperatures that restrict (18°C, black lines) or permit (32°C, gray lines) PTX expression.

Figure 2

Inhibition of heterotrimeric G o signaling reversibly reduces the amplitude of ethyl acetate evoked EAG responses. For all fly strains, a 10-4 dilution of ethyl acetate was used to evoke EAG responses. (A) EAG responses from Gal80ts20/+; UAS-PTX/+ and Or83b-Gal4/+ control strains do not decrease (p > 0.5) at 32°C compared to 18°C. Gal80ts20/Or83b-Gal4; UAS-PTX/+ flies have a significantly (p < 0.0001) higher EAG amplitude in the absence of PTX expression before heat induction (18°C) or after recovery from heat induction (18°C#) than in the presence of PTX expression (32°C). (B) EAG responses from Or83b-Gal4; UAS-rtTA/Teto-PTX flies are significantly (p < 0.0001) higher in the absence of PTX expression (dox -) than in the presence of PTX expression (dox +). EAG responses from flies that express PTX-insensitive Go (PiGo) in ORNs (Teto-PTX, UAS-PiGo/+; Or83b-Gal4, UAS-rtTA) are not different (p > 0.7) whether PTX expression in ORNs is induced (dox +) or uninduced (dox -). For each genotype and treatment, at least 12 EAG recordings from minimum 6 flies were analyzed. Asterisks denote a significant (p < 0.05) change. All values are mean ± S.E.M.

To confirm that PTX suppressed EAG amplitude, PTX was conditionally expressed in ORNs by combining the Gal4/UAS and tetracycline (Tet)-inducible Tet-On transactivator (Tet-On TA) systems [14]. Or83b-Gal4 was used to drive expression of UAS-rtTA (reverse tetracycline transactivator) in ORNs. In the presence of the tetracycline analog doxycycline, rtTA binds to the tet-operator (teto) and activates transcription of the teto-PTX transgene. Upon addition of doxycycline, PTX expression suppressed (p < 0.0001) EAG amplitude by ~40% (Fig 2B). To insure that PTX suppressed EAG amplitude by inhibiting Go, a PTX insensitive Goα (PiGo) was expressed along with PTX in ORNs. Doxycycline-induced PTX expression did not affect (p > 0.7) EAG amplitude in flies expressing PiGo in ORNs, demonstrating that PiGo completely rescued the action of PTX on endogenous Go (Fig 2B). These results map the effects of PTX to Go and confirm that Go signaling contributes to olfactory responses.

To investigate the effect of PTX on EAG dynamics, we looked at fall time constant (τf) as a measure of the termination kinetics of EAG responses. Fall time constant is the time necessary to recover one-third of the maximal EAG amplitude after stimulation. This parameter is independent of amplitude, and unlike amplitude τf remains relatively unaffected by small changes in electrode placement [15]. Upon stimulation for 500 ms with 10-4 ethyl acetate, τf was significantly (p < 0.01) lowered in the Gal80ts20/Or83b-Gal4; UAS-PTX/+ flies at 32°C compared to that at 18°C, whereas the two control strains showed no effect (p > 0.8) of temperature on τf (Fig 3). For a given odorant, τf decreases if either the concentration of the odorant or its delivery duration is reduced [15]. Inhibition of Go resulted in faster termination kinetics typically seen in control flies upon application of a 10-fold lower dose of odorant (Additional file 2). Since inactivation of Go shortened τf, it can be argued that transduction of odor-information in the antenna was impaired in absence of Go. Our observation that Go is needed for the persistence of the electrophysiological response in vivo corroborates the in vitro results that implicate G-protein mediated signal amplification in prolonged odor signaling [2].
Figure 3

G o activity is required for the perdurance of EAG responses. For each fly strain, a 10-4 dilution of ethyl acetate was used to evoke EAG responses. The EAG fall time constant in Gal80ts20/+; UAS-PTX/+ and Or83b-Gal4/+ control strains is not different (p > 0.8) at 18°C and 32°C. Gal80ts20/Or83b-Gal4; UAS-PTX/+ flies have a significantly (p < 0.01) longer EAG fall time constant in the absence of PTX expression (18°C) than in the presence of PTX expression (32°C). For each genotype and temperature, at least 8 EAG recordings from minimum 4 flies were analyzed. Asterisks denote a significant (p < 0.05) change. All values are mean ± S.E.M.

Odor-induced EAG responses are thought to mainly consist of the summation of receptor potentials of many ORNs in close proximity to the recording electrode [16]. However, it is difficult to correlate EAG responses with single cellular processes that occur when individual ORNs respond to odorants. The limited resolution of EAGs can be overcome by recording single unit responses from individual sensilla. In contrast to EAG responses, single unit recordings consist of spikes that represent extracellularly recorded action potentials of individual ORNs in the sensillum [17]. To investigate the role of Go at the level of single cell physiology, we performed single-sensillum recording on ab1 sensilla whose 'A' neuron (i.e. the neuron producing the largest 'A' spike) is known to robustly respond to ethyl acetate [18]. Expression of PTX significantly (p < 0.0001) reduced the ethyl acetate-evoked firing frequency of ab1A spikes (Fig 4). However, the spontaneous firing frequency did not (p > 0.2) change, indicating that inactivation of Go did not alter the physiology of uninduced resting membrane (Additional file 3). The same sensillum houses the CO2-sensing ab1C neuron [1820], which does not express Or83b-driven PTX. CO2-induced single unit responses are not affected by Or83b-driven PTX in ab1C neurons (Fig 4), thus confirming the specificity of our gene expression system. The reduction in ethyl acetate induced spike frequency was not a mere physical response caused by increase in temperature because the two control strains did not show any decrease (p > 0.5) in firing frequency in response to increased temperature. Inhibition of Go signaling lowered the odor-induced frequency of ab1A spikes and odor-evoked EAG response by an equivalent amount, i.e., 40-45% reduction in response. Taken together, these results reveal that Go plays an important role in olfactory reception within the Drosophila ORNs.
Figure 4

G o inhibition reduces odor-evoked firing frequency. For each fly strain, CO2 and a 10-4 dilution of ethyl acetate were used to evoke spike activity from ab1C or ab1A neurons respectively. Spike frequency in Gal80ts20/+; UAS-PTX/+ and Or83b-Gal4/+ control strains do not decrease (p > 0.5) at 32°C compared to 18°C. Gal80ts20/Or83b-Gal4; UAS-PTX/+ flies have a significantly (p < 0.0001) lower ethyl acetate evoked ab1A spike frequency in the absence of PTX expression (18°C) than in the presence of PTX expression (32°C), whereas CO2-induced single unit responses in the ab1C neuron was not unaffected (p > 0.8). For each genotype and temperature, responses from at least 8 ORNs from minimum 4 flies were analyzed. Asterisks denote a significant (p < 0.05) change. All values are mean ± S.E.M.

To determine whether inhibition of Go signaling impairs olfactory responses only at certain concentrations of ethyl acetate, we recorded EAG responses in both PTX expressing and PTX non-expressing Gal80ts20/Or83b-Gal4; UAS-PTX/+ flies exposed to various concentrations of ethyl acetate (Fig 5A). PTX was found to repress EAG responses over a 1000-fold range of stimulus intensities (p < 0.0001); although the degree of repression was slightly higher at high concentrations of ethyl acetate. This effect was in contrast with the odor-intensity dependent effect of dGq3RNAi in behavioral response of Drosophila to odors [21]. Odor sensitivity was compared by noting the increase in odor concentration that is needed in Go-compromised flies to elicit EAG responses as high as that in flies with unaffected Go(see Materials and Methods). Comparison of the two dose-response curves reveals that PTX mediated suppression of EAG response is associated with a ~470 fold difference in sensitivity to ethyl acetate (Fig 5A).
Figure 5

G o signaling is required for normal EAG responses to diverse odorants. (A) EAG responses evoked by four different concentrations of ethyl acetate (EA) in Gal80ts20/Or83b-Gal4; UAS-PTX/+ flies are significantly (p < 0.0001) higher in the absence of PTX expression (18°C) than in the presence of PTX expression (32°C). (B) EAG responses evoked by a 10-4 concentration of ethyl acetate (EA), a 10-4 concentration of isoamyl acetate (IAA), a 10-4 concentration of cyclohexanone (CH), a 10-4 concentration of 4-methylcylcohexanol (MCH), and a 10-3 concentration n-butanol (BUT) in Gal80ts20/Or83b-Gal4; UAS-PTX/+ flies are significantly (p < 0.0001) higher in the absence of PTX expression (18°C) than in the presence of PTX expression (32°C). For a given genotype, odor concentration, and temperature, at least 8 EAG recordings from minimum 4 flies were analyzed. Asterisks denote a significant (p < 0.05) change. All values are mean ± S.E.M.

We next determined whether Go contributes to the detection of odorants by other classes of sensilla. We chose a small panel of odorants, which included two acetates (ethyl acetate, isoamyl acetate) perceived by basiconic sensilla, one ketone (cyclohexanone) known to activate a single class of coeloconic sensilla, an alcohol (4-methyl-cylcohexanol) that is detected by trichoid and coeloconic sensilla, and another alcohol (n-butanol) that is detected by basiconic and coeloconic sensilla [18, 2224]. Our odor panel contained both attractants (e.g. ethyl acetate at 10-4 concentration) and repellents (e.g. 4-methyl-cyclohexanol at 10-4 concentration). EAG recordings revealed that PTX expression significantly (p < 0.0001) repressed EAG amplitudes to all five odorants tested (Fig 5B). In each case, the EAG amplitude was reduced by 38 ± (+/- 5) percent. These results suggest that Go plays a role in olfactory signaling across multiple classes of sensilla independent of odor identity or concentration.

Our results show that sensory signals from five odorants, including ethyl acetate, are modulated via Go signaling. These findings support the possibility that a single odorant may activate multiple transduction pathways since previous studies showed that Gq is needed for optimal responses to isoamyl acetate, ethyl acetate and butanol [3, 21]. Activation of Drosophila OR cation channel function by multiple odorants implies that both OR channel function and G-protein signaling are required for optimal responses to a given odor [1]. It is possible that odor bound ORs directly activate Go and Gq, thus reinforcing and optimizing the ORN response by modulating second messenger levels.

Conclusion

Our results demonstrate that Go is required for maximal physiological responses to a diverse group of attractive and aversive odorants in Drosophila. Given that diminished physiological responses to odors persist in the absence of Go signaling, it is likely that OR channel function, along with G-protein signaling, are required for optimal physiological responses to odors.

Methods

Generation of transgenic flies

The Gal80ts20; UAS-PTX.16 and UAS-rtTA901 were both previously described [10, 14]. The Pertussis insensitive Goα was generated through the incorporation of the cysteine351 to isoleucine mutation in the wild type GM1620 Goα 47A cDNA by PCR. This pertussis insensitive Goα cDNA (PiGo) was cloned into the pPUAST vector [25]. The tet0-Pertussis toxin construct was assembled in two parts. The PTX coding region was cloned by PCR from pPUAS-PTX to include a Pst I site at the 5' end and an Eco RI site at the 3' end [10]. This construct was directly cloned into the Pst I and Eco RI sites of the pUSC1.0 vector [26]. The SV40 polyadenylation sequence from pBRETU was subsequently cloned as an Eco RI fragment behind the PTX coding sequences to generate the pPteto-PTX vector [27]. Transgenic PUAS-PiGo and Pteto-PTX flies were generated through direct embryo injection by Rainbow Transgenetic Flies, Inc. (Newbury Park, CA).

Electrophysiological recording techniques

EAG and single-sensillum recording experiments were performed as previously described [28, 29]. Recordings were carried out during the middle of the day on 2-5 day old flies raised at 18°C. Temperature sensitive GAL80ts20 was inactivated by placing flies at 32°C for 18 hours. Heat-treated flies were then kept at 18°C for 24-48 hours for recovery. The Tet-On system was activated by feeding flies a 2% sucrose solution containing 2 mM doxycycline overnight. Dilutions of all odorants except CO2 were made in mineral oil. Odors were delivered for approximately 500 ms. At least eight EAG or single unit recordings from at least four different flies were analyzed for each data point. To quantify spike frequency, recordings from 10 different ORNs from at least 4 different flies were analyzed. Spikes were manually sorted and spontaneous frequency was not subtracted from the odor-induced net response. Statistical significance with respect to pairwise comparison was calculated using Student's t-test, and multiple means were compared by one-way ANOVA. The Bonferroni test was used for post hoc analyses. The PTX-induced change in sensitivity to ethyl acetate was calculated using a fitted linear equation (EAG amplitude = -2.45 × negative log dilution of ethyl acetate + 22.3) derived from the dose response curve from PTX non-expressing flies. A response of 8.4 mV to a 10-3 dilution of ethyl acetate in PTX expressing flies equates to a 10-5.67 dilution of ethyl acetate in PTX non-expression flies, or a ~470-fold reduction in stimulus concentration to produce the same response.

Abbreviations

ORNs: 

Olfactory receptor neurons

ORs: 

odorant receptors

EAG: 

electroantennogram

EA: 

ethyl acetate

PTX: 

pertussis toxin

Go

heterotrimeric G-protein (o)

tet: 

tetracycline

dox: 

doxycycline

PiGo

PTX insensitive Goα

τf

fall time constant

teto

tet-operator.

Declarations

Acknowledgements

We thank John Tower (University of Southern California) for providing plasmids and fly strains. We also thank Laura Bertrand and Lingzhi Liu for technical assistance, Gladys Ko for help with data analysis, and Jerry Houl for transporting fly strains from University of Houston to Texas A&M University. This work was supported by the Texas Advanced Research Program Grant # 003652-0060-2007 to GR and by funds from Texas A&M University to PEH.

Authors’ Affiliations

(1)
Department of Biology and Center for Biological Clock Research, Texas A&M University
(2)
Department of Biology and Biochemistry, University of Houston

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© Chatterjee et al; licensee BioMed Central Ltd. 2009

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.